This patent relates to fluid dispensing cartridges for automated dispensing systems, such as ink dispensing cartridges used with high-speed lithographic printing presses. More specifically, this patent relates to an improved plunger that helps control the reflection of ultrasonic signals used to sense the position of the plunger as it moves within the cartridge tube.
Typical lithographic ink cartridges comprise a hollow cylindrical body, a plunger and a dispensing fitment. The cylindrical body holds a supply of extrudable ink or varnish and has a dispensing end and a plunger end. The dispensing end is sealed with the dispensing fitment, which typically includes a nozzle for directing the flow of the ink. The plunger end is sealed by the plunger, which moves within the cylindrical body in response to pneumatic or other pressure to extrude ink out the nozzle.
Monitoring the level of ink in the cartridge is desirable for two reasons. First, it is necessary to know when the ink cartridge is empty and requires replacing. Second, it is desirable to know how much ink is being used for a particular printing job.
One method for determining the level of the ink in the cartridge involves sending a series of ultrasonic signals toward the plunger from a source mounted in the top of the automatic dispensing system and receiving reflected signals back. The reflected signals are processed to determine the level of the plunger and, concomitantly, the level of the ink in the cartridge body.
Conventional plungers for lithographic ink cartridges are made of plastic and are substantially cup-shaped. The plunger typically comprises a planar bottom wall and a sidewall extending upward from the periphery of the planar bottom wall. As the plunger is forced through the cartridge body by pneumatic pressure, the planar bottom wall pushes against the ink, causing it to be expelled through the dispensing fitment nozzle.
Due to the high viscosity of the ink and the tendency of certain fast drying inks to adhere the plunger sidewalls to the cartridge body, very high pressures are sometimes needed to advance the plunger. It has been found beneficial to mold stiffening ribs into the backside of the planar bottom wall to minimize twisting and distortion of the plunger when it is being subjected to high pressures.
Unfortunately, these stiffening ribs can interfere with the reflection of the ultrasonic signals, preventing an accurate reading of the ink level. This is because the height of the ribs often varies along their length and in places exceeds that which would allow an accurate reading of the plunger position using ultrasonic signals. A further problem is caused if the ribs are not identical. To alleviate the problems presented by the stiffening ribs, a disk having a flat reflecting surface may be placed behind the plunger, but this solution entails added cost.
The conventional xe2x80x9cflatxe2x80x9d plunger described above has an additional disadvantage in that it has a tendency to trap air inside the cartridge body. After the cartridge has been filed with ink and the plunger inserted into the cartridge, air can become trapped underneath the plunger. Ink contacting the air can dry out over time. If the dried ink is expelled from the cartridge, it can create printing defects such as hickies.
The problem of air entrapment can be minimized or eliminated by using a plunger having a convex surface facing the ink, such as that disclosed in Lawson et al. U.S. Pat. No. 6,419,351. The convex shape facilitates the flow of air away from the center of the plunger and toward the cartridge body wall when the plunger is inserted into the filled cartridge and pressed against the ink. To further aid the flow of air away from the center of the plunger, the bottom surface of the plunger may be textured with bumps, nubs, ridges, grooves or any other type of projection or indentation capable of defining channels through which air can flow when the plunger comes into contact with the ink.
The convex plunger disclosed in the ""351 patent has a corresponding concave upper surface. When such a plunger is used with ultrasonic sensors, the concave upper surface cancels out the ultrasonic signal, causing an error reading. To alleviate this problem, the convex plunger may be molded with a flat backside to better reflect ultrasonic signals, but this solution entails added material and cost. Furthermore, molding a convex plunger with a flat backside results in an unacceptably longer cycle (manufacturing) time due to the additional time needed to cool the extra thermoplastic material contained in the newly molded plunger.
Thus it is an object of the present invention to provide a convex plunger that enables proper reflection of ultrasonic signals used to sense the position of the plunger as it moves within the cartridge tube.
Further and additional objects will appear from the description, accompanying drawings, and appended claims.
The present invention is an improved plunger for fluid dispensing cartridges that helps control the reflection of ultrasonic signals used to sense the position of the plunger as it moves within the cartridge tube. In one anticipated application, the plunger is used in ink cartridges for high-speed lithographic printing systems. The plunger bottom wall has a convex bottom (fluid facing) surface and a top surface having a reflective area comprising one or more flat, horizontal surfaces. In the preferred embodiment, the horizontal surfaces are concentric steps of equal height.